Although these rules are simple to apply, there is no guidance in F4 with respect to where the reinforcement should be added. The implications are that some of these split-tees will be connected to other pipelines, and possibly be buried, thereby inducing significant system loadings. Using the area replacement philosophy, it is possible to have a design with a branch thickness that just meets the pressure requirements but makes no allowance for system loading. Therefore, there is a strong possibility that subsequent system loadings on the branch may overstress the component and possibly cause the component to fail by either plastic collapse, incremental plasticity (ratchetting) or fatigue. This is one issue that F4 and the manufacturer’s could address at the design stage if the magnitude of the system loading for which the split-tee is to be designed for was known. However, this information is not known until a flexibility analysis of the layout has been performed. A fitness-for-purpose assessment method of split-tees is therefore required to determine the significance of the system loads, once the design of the split-tee to internal pressure has been confirmed and once the flexibility analysis results are available. 2.4
Avoidance Of Post Weld Heat Treatment (PWHT) For large diameter (>36in), high pressure (>70bar) gas transmission pipelines,
the area replacement methodology requires large wall thicknesses of the run and branch pipes, typically 50mm.
The ASME code would require welds of these
thicknesses to undergo PWHT. The F4 specification requires the fitting ends to be locally reduced by chamfering to a thickness (Figure 4) which is twice the carrier pipe wall thickness at the locations where the circumferential fillet welds are to be made. It is considered that, not only is this used to control the welding operation by reducing the number of weld deposits, but also as a method of ensuring the fillet welds avoid the requirement of any PWHT. By reducing the throat area of the weld, for large diameter, high pressure pipelines, these welds can become highly stressed due to the operational loadings.